1. Field of the Invention
The present invention relates to an exhaust emission control device for internal combustion engine, and more specifically, to a technique for improving exhaust gas purification efficiency with use of a three-way catalyst.
2. Description of the Related Art
In general, three-way catalysts are widely used as exhaust gas purifying catalysts for vehicular internal combustion engines. The three-way catalysts are constructed so that they can approximate the air-fuel ratio of exhaust gas to the theoretical air-fuel ratio (stoichiometric ratio), thereby optimizing the oxidation of HC and CO and the reduction of NOx and accelerating exhaust gas purification.
Recently, there has been developed a three-way catalyst that has a so-called oxygen (O2) storage component (OSC). The OSC is a function that is intended to maximize the performance of a three-way catalyst. The catalyst with the OSC stores oxygen in lean air-fuel ratio operation to form a near-stoichiometric catalyst atmosphere temporarily, thereby purifying NOx. Thereafter, the catalyst releases the stored oxygen in rich air-fuel ratio operation, thereby accelerating purification of HC and CO.
Another three-way catalyst that has a so-called CO storage component (COSC) has also been developed (Jpn. Pat. Appln. KOKAI Publication No. 2002-89250). The catalyst with the COSC stores a reducing agent, such as CO, in rich air-fuel ratio operation, and thereafter, releases the stored CO in lean air-fuel ratio operation, thereby accelerating purification of NOx.
A three-way catalyst that combines the OSC and the COSC has also been developed.
With use of the three-way catalyst of this type, air-fuel ratio modulation control is carried out between lean and rich air-fuel ratio operations, and storage and release of oxygen and CO or the like are alternately repeated. Thus, HC and CO, along with NOx, can be continuously purified with high efficiency.
When a vehicle is accelerated or decelerated, supply of a fuel to an internal combustion engine sometimes cannot follow a command value, and transportation of the fuel is liable to be delayed. If the air-fuel ratio modulation control is carried out with an exhaust air-fuel ratio set so that the three-way catalyst having the OSC and the COSC is in an optimum state, in this case, an error is temporarily caused between the set exhaust air-fuel ratio and an actual exhaust air-fuel ratio. In consequence, the lean and rich air-fuel ratio operations are unbalanced, and the exhaust air-fuel ratio may possibly overshift to the lean or rich air-fuel ratio side.
If the balance between the lean and rich air-fuel ratio operations is broken, the state of the three-way catalyst is not optimum any longer. If the exhaust air-fuel ratio overshifts to the lean air-fuel ratio side, for example, an oxygen-rich state is established. Accordingly, the OSC ability is exceeded, and the catalyst is entirely in an oxidative atmosphere. Thus, a so-called NOx spike is generated, and the NOx purification rate temporarily lowers by a large margin. If the exhaust air-fuel ratio overshifts to the rich air-fuel ratio side, on the other hand, a CO-rich state is established. Accordingly, the COSC ability is exceeded, and the catalyst is entirely in a reducing atmosphere. Thus, a so-called HC. CO spike is generated, and the HC and CO purification rates temporarily lower by a large margin.